1. Field of the Invention
The present invention relates generally to cement compositions utilized in the completion and repair of wells, and more particularly, to set retarded ultra fine cement compositions and methods of their use.
2. Description of the Prior Art
In cementing operations such as those carried out in completing and repairing oil, gas and water wells, a hydraulic cement composition is prepared by mixing a hydraulic cement with water and other additives, the composition is placed into a subterranean zone to be cemented and allowed to set into a hard substantially impermeable mass therein.
In the operation of wells used in the recovery of fluids from or the introduction of fluids into subterranean formations problems relating to the unwanted passage of fluids and/or fine solids into or from undesirable locations in the formation or well bore sometimes occur. This unwanted passage of fluids and/or fine solids can severely disrupt or terminate the desired operation of a well.
The problems involving the unwanted passage of fluids referred to above, ordinarily involve the movement of fluids, such as oil, gas or water through very small undesirable openings. These problems are not unique and the solutions have traditionally involved apparatus, methods and compositions adapted to cover, seal or to otherwise plug the openings to thereby terminate the unwanted passage of fluid through the openings. The openings referred to above include: holes or cracks in well casing; spaces such as holes, cracks, voids or channels in the cement sheath deposited in the annular space between the formation face and well casing; very small spaces--called microannuli--between the cement sheath referred to above, and the exterior surface of the well casing or formation; and permeable spaces in gravel packs and formations.
It is clear that holes or cracks in well casing and/or cement sheath can permit the unwanted and therefore uncontrolled passage of fluids therethrough. Sometimes, of course, holes are deliberately made in casing and sheath by a known process called perforating in order to permit the controlled recovery of fluid from a formation or to permit the controlled introduction of fluid into a formation. The sealing or plugging of such holes or cracks, whether or not made deliberately, has been conducted by attempts to place or otherwise force a substance into the hole or crack and permitting it to remain therein to thereby plug the opening. Naturally, the substance will not plug the opening if it will not enter the opening. If the substance does not fit then, at best, a bridge, patch, or skin may be formed over the opening to produce, perhaps, a temporary termination of the unwanted fluid flow.
Substances used in methods to terminate the unwanted passage of fluids through holes or cracks in casing and/or sheath have been compositions comprised of hydraulic cement, wherein the methods employ hydraulic pressure to force a water slurry of the cement into the cracks and holes wherein the cement is permitted to harden. These methods are variously referred to in the art as squeeze cementing, squeezing or as squeeze jobs. The success of squeezing hydraulic cement into such holes and cracks is among other factors a function of the size of the hole relative to the particle size of the cement as well as the properties of the slurry. As mentioned earlier, if the particle size of the cement is greater than the crack width, the cement will not enter and at best a patch instead of a plug is the result. A problem therefore is to substantially reduce cement particle size without reducing the hardening and strength characteristics of hydraulic cement.
During the construction of a well it is known to place a volume of a water slurry of a hydraulic cement into the annular space between the walls of the well bore and the exterior of the casing wherein the cement is permitted to solidify to thereby form an annular sheath of hardened cement. The objective of the sheath, the construction of which is referred to as primary cementing, includes physical support and positioning of the casing in the borehole and prevention of unwanted fluid (liquid and gas) migration between various formations penetrated by the well bore. If, for some reason, the hardened sheath contains spaces such as voids, cracks or channels due to problems involved in the placement of the slurry it is clear that the sheath may not be capable of providing the desired objectives. Accordingly, by employing known techniques to locate the voids, channels or cracks, a perforation penetrating the spaces can be made in the casing and sheath and cement then squeezed into the spaces via the perforation so as to place the sheath in a more desirable condition for protecting and supporting the casing and providing fluid flow control. As mentioned earlier, the success of the squeeze job is at least a function of the size of the space or spaces to be filled relative to the particle size of the cement.
Another problem incidental to the formation of the cement sheath revolves about the occasional failure of the sheath to tightly bond to the exterior wall of the casing or the interior of the borehole. This failure can produce a very thin annular space called a microannulus between the exterior wall of the casing and the sheath or the sheath and the borehole. For the reasons already discussed, it is important to place a substance, such as a hydraulic cement, in the microannulus to enable the sheath to fully provide the intended benefits. Again, as stated above, the success of squeezing cement into a microannulus space is dependent upon the relative size of the space and the particle size of the cement.
The solid portions of some producing formations are not sufficiently stable and therefore tend to break down into small pieces under the influence of the pressure difference between the formation and the well bore. When fluid, such as oil or water, flows under the influence of the pressure difference from the formation to the well bore the small pieces referred to above can be carried with the fluid into the well bore. Over a period of time, these pieces can build up and eventually damage the well and associated equipment and terminate production. The art has solved this problem by placing in the well bore a production aid which is referred to in the art as a gravel pack. A gravel pack is usually comprised of a mass of sand within the interior of a well. The sand bed completely surrounds a length of tubular goods containing very narrow slots or small holes; such goods are sometimes referred to as slotted liners or sand screens. The slots or holes permit the flow of fluid therethrough but are too narrow to permit the passage of the sand. The slotted liner or sand screen can be connected through a packer situated up-hole of the gravel pack to production tubing extended from the wellhead. The gravel pack ordinarily consists of siliceous material having sand grains in the range of from about 10 to about 100 mesh.
The gravel pack, which can be situated in the casing in the perforated interval, traps the small pieces of formation material, for convenience herein referred to as formation fines or sand, which flows from the formation with the fluid through the perforations and into the gravel pack. Accordingly, neither formation sand nor gravel pack sand penetrates the slotted tubing and only fluid is permitted to pass into the tubular goods.
The above expedient performs nicely until undesired fluid begins to penetrate the gravel pack from the formation. At that point the flow of undesired fluid, such as water, must be terminated preferably in a way which will not necessitate removal of the gravel pack.
The problems referred to above uniformly deal with the unwanted passage of materials into or from very small undesirable openings in a well, including the cement sheath constructed during a primary cementing procedure. Still another problem involved in the construction and repair of wells involves the primary cementing procedure itself.
Primary cementing, as described above, is conducted during the construction of a well and involves the placement of a volume of a slurry of a hydraulic cement and water into the annular space between the walls of the well bore and the exterior of primary casing such as conductor pipe, surface casing, and intermediate and production strings. The slurry is permitted to solidify in the annulus to form a sheath of hardened cement, the purpose of which is to provide physical support and positioning of the casing in the well bore and to isolate various formations penetrated by the well bore one from another.
A problem encountered during primary cementing is centered upon the weight (that is the density) of the slurry itself. In certain circumstances the hydrostatic pressure developed by a column of slurry overcomes the resistance offered by a formation in which case the formation fractures or otherwise breaks down with the result that a portion of the slurry enters the formation and the desired sheath is not formed. The formation breakdown thus occurs prior in time to development of sufficient rigidity or hardening of the cement to enable it to be self-supporting.
One solution has been to reduce the density of the slurry so that the pressure developed by the required slurry height will not exceed the ability of the formation to resist breakdown. This expedient can result in sheaths having physical deficiencies such as reduced strength or increased permeability or both. Another solution has been to reduce the weight of the slurry while maintaining density by reducing the quantity of slurry pumped in a single lift or stage to thus reduce the height of slurry. This expedient requires several separate stages in order to produce the required sheath length. Time must pass between stages in order to permit previous stages to develop strength sufficient to support the weight of succeeding stages. The time expended waiting on cement to set is lost time in the process of constructing the well.
Still another problem involved in the operation of wells revolves about the unwanted movement of water via cracks and fractures in the subterranean formation, whether naturally occurring or deliberately produced, from the formation into the well bore. Terminating this water movement may require remedial efforts other than those referred to previously which feature plugging perforations, holes, cracks and the like in casing, cement sheath and gravel packs, all of which occur within the confines of the well bore itself.
Recently, a better solution to all of the problems mentioned above has been developed and utilized successfully. The solution involves the use of ultra fine hydraulic cement compositions and is described in detail, for example, in U.S. Pat. No. 5,086,850 issued on Feb. 11, 1992. The ultra fine cement compositions and methods described in U.S. Pat. No. 5,086,850 have been utilized successfully in primary cementing, squeeze cementing and the other forms of well cementing mentioned. However, a problem which has persisted in the use of ultra fine cement compositions occurs when the compositions are subjected to high temperatures, i.e., temperatures in the general range of from about 140.degree. F. to about 250.degree. F. In carrying out cementing operations of the type described above in deep hot wells, the ultra fine cement composition is heated to temperatures in the above mentioned range which, heretofore, has often caused premature gelling of the cement composition and the very severe problems attendant thereto. While prior art set retarders have been included in the ultra fine cement compositions, e.g., lignosulfonates, predictable retardation of the compositions at temperatures above about 140.degree. F. has not been obtained.
Thus, there remains a need for a set retarded ultra fine cement composition which remains pumpable for a predictable period of time and for methods of using such composition in remedial cementing and primary cementing operations.